1. Field of the Invention
This disclosure relates generally to a gas analyzer comprising a light source, a sampling chamber and at least one detector.
2. Description of Related Art
Conventionally it has been adequate enough to measure a gas concentration at steady states only, however, continuous, real time gas analyzing is becoming more and more important in different kind of applications. To measure fast gas concentration changes during different transients also a response time of an analyzer should be quick enough, both in an electric and pneumatic sense. It is also desired to measure more than one gas component of a gas mixture at once.
A breath by breath concentration measurement of different respiratory gases is one of the most demanding forms of a gas measurement. A human can breath spontaneously up to 200 breaths per minute, but a patient can be ventilated mechanically close to thousand breaths per minute as well. However, the high frequency ventilation is more like a vibration and a diffusion of gas molecules and the outcome looks more like a constant gas concentration rather than a breath to breath alternating signal. The gas concentration of CO2, that the patient produces, is normally around five volume percents at an end of an expiration, thus with a healthy patient the concentration of CO2 varies from 0 to 5 volume percents between the inspiration and the expiration. A time used for the inspiration is usually shorter, only one third, compared to a time used for the expiration, which is two thirds of the breathing cycle. This means that the possible steady state time of the concentration at the end of the inspiration is smaller compared to that of the expiration. As a frequency of breathing increases the steady state time of the gas concentration also decreases proportionally. In addition a breathing system mechanics starts to affect shortening the time even more although the gas concentration change between the inspiration and the expiration at an alveoli deep in the lungs is fast.
The gas concentration curve drawn according to the change of the inspiration and the expiration would look like a square wave inside bronchiole just by the alveoli. However, since the inspired and expired gases mix as they travel back and worth within the inspiration and the expiration through the dead space of bronchioles, bronchi, bronchus, trachea, endotracheal tube, part of the breathing circuit and finally through different connectors in to the analyzer located outside the patient the concentration curve at the analyzer input would look more like a filtered square wave with rounded corners. As the frequency of breathing is increased the breathing volume decreases proportionally, if the ventilator working pressure is maintained constant, which in turn increases the mixing of the inspired and expired gases in the dead space even more, since smaller amounts of gases move inside relatively larger dead space.
Gas samples of respiratory gases are sucked from the breathing circuit in to the analyzer through a sample gas tube. Inside the analyzer gas samples are processed to get a continuous, real time gas concentration waveform, of course with a small processing delay. At this point, especially with higher breathing frequencies, the waveform looks more like a sine wave because of the further mixing of inspiratory and expiratory gas samples inside the analyzer. If more gases in addition to for example CO2 is measured the processing of gas samples may mix the gas even more also worsening the signal. It is also desirable to keep the sample gas flow as small as possible so that the actual process that is being measured is not disturbed too much. Lowering of the sample gas flow increases the analyzer response time dramatically since mixing of sample gases, caused by diffusion, turbulences and inertia near the walls in accordance with a laminar flow, increase as they spend more time flowing through the sample gas tube in to the analyzer, but also inside the analyzer as the sample is processed. Thus noticing what is explained above with conventional techniques used to measure gas concentrations in respiratory care it is possible to measure different breathing gas concentrations continuously, in real time, up to 20-40 breathes per minute only depending on the measured gas.
The response time of the analyzer can be expressed in terms of a rise time and a fall time that helps to understand better the functioning of the analyzer in time sense. The rise time is the time in which the gas analyzer output signal changes its state from 10% to 90% level of its total signal change as the gas concentration in the analyzer input changes from the constant lower gas concentration level to the constant higher gas concentration level.
The analyzer rise/fall time is basically a combination of the rise/fall times of an electrical circuitry and a pneumatic system. The analyzer rise/fall time can be described with a simplified equation τ=√{square root over (X2+Y2)}, in which X is a total electrical rise/fall time and Y is a total pneumatic rise/fall time. Normally some material in connection with the measured gases cause additional increase into rise/fall time as they may absorb and emit gas molecules. To mention other things also the viscosity of gases may increase or decrease rise/fall time. The total electrical rise/fall time is similarly expressed with an equation τ=√{square root over (X12+X22+ . . . )}, where X1, X2 . . . are rise/fall times of each electrical components such as a detector that transforms an infrared radiation in to an electrical voltage proportional to the gas concentration or electrical filters used to filter a noise from the signal produced by the detector or amplifiers etc. A length, shape and smoothness of a flow path, through which the sample gas flows from a location where the sample was taken in to a place where it is analyzed, causes increase in the pneumatic rise/fall time. The total pneumatic rise/fall time can be expressed with an equation τ=√{square root over (Y12+Y22+ . . . )}, where Y1, Y2 . . . are rise/fall times of each pneumatic components such as mechanical connections that are step like changes with an additional space along the flow path or just sharp corners or some kind of flow barriers such as filters etc. Usually the pneumatic design of the analyzer system is more dominant thus the effect of the electrical rise/fall time X is smaller than the effect of the pneumatic rise/fall time Y in the equation of the analyzer rise/fall time.
The flow speed of the sample gas through the pneumatic system has an influence on the rise/fall time also. The longer the sample gas travels through the small tubing and cavities inside the analyzer the more the gas samples containing different gas concentrations mix up. On the other hand in many cases it would be desirable to “steal” as little sample gas as possible from the primary system to be analyzed at the analyzer. Such a low sampling flow the gas analyzer is challenging to implement since the sensor rise/fall time increase as the sample flow is decreased. This in turn makes the gas concentration measurement at higher transition frequencies even more difficult.
A sampling chamber, where the sampled gas is analyzed and which enables the concentration measurement of even seven different gas components from the one gas mixture, is one of the most dominant pneumatic components which increases the pneumatic system rise/fall time of the gas analyzers.
A cross sectional view of a conventional sampling chamber 1 inside a housing 2 is shown in FIG. 4 from a direction of a radiation source (not shown) to a detector (not shown). The sampling chamber 1 is cylindrical when looked from the direction of radiation source, but rectangular when looked from aside. An inlet tube 3 and an outlet tube 4 are straight. The sample gas flows to the sampling chamber 1 through inlet tube 3 in to the direction of dotted lines that show how the gas flows through the sampling chamber and out from the sampling chamber through the outlet tube 4. The step like change in the flow path from the inlet tube 3 in to the sampling chamber 1 causes a turbulence inside the sampling chamber aside exit of the inlet tube (not shown in the figure), which mixes up the gas flow. The gas flowing through the cavity causes a strong turbulence or curl in to the middle of the sampling chamber also, which mixes up the new gas entering the chamber with the gas already inside the chamber circulating within the curl. This slows down the rise/fall time of the sampling chamber considerably. A reason for the turbulence is the fact that when the gas sample is discharged from the narrow inlet tube 3 to a large volume of the sampling chamber 1 gas molecules of the gas sample try if allowed to fill a larger volume and obtain more room inside the sampling chamber. Then the flow cannot be any more laminar, which is desired for keeping the gas concentration also inside the sampling chamber as unchanged as possible corresponding to a real situation when the sample was actually taken. Since the sampling chamber is just the place where the radiation traverses through the sample gas and traversed radiation is finally received by the detector for analysis, turbulences especially inside the sampling chamber have a very negative impact on analysis results.